Higher temperature solution processes for olefin polymerization are highly desirable due to the increased throughput, decreased energy necessary for devolatization and decreased fouling that these higher temperatures afford. Although Ziegler-Natta catalyst systems can be run at high temperatures commercially, these catalysts suffer from poor efficiency and poor comonomer incorporation at elevated temperatures. In addition, polymers produced from Ziegler-Natta catalysts at elevated temperatures have broadened molecular weight distributions, which limits their suitability for use in many applications. Conventional Ziegler-Natta catalysts are typically composed of many types of catalytic species, each having different metal oxidation states and different coordination environments with ligands. Examples of such heterogeneous systems are known and include metal halides activated by an organometallic co-catalyst, such as titanium chloride supported on magnesium chloride, activated with organoaluminum and organoaluminumhalide cocatalysts. Because these systems contain more than one catalytic species, they possess polymerization sites with different activities and varying abilities to incorporate comonomer into a polymer chain. The consequence of such multi-site chemistry is a product with poor control of the polymer chain architecture. Moreover, differences in the individual catalyst site produce polymers of high molecular weight at some sites and low molecular weight at others, resulting in a polymer with a broad molecular weight distribution and a heterogeneous composition. Due to this heterogeneity, mechanical and other properties of the polymers may be less than desired.
More recently, catalyst compositions based on well defined metal complexes, especially transition metal complexes such as constrained geometry catalysts (CGCs), metallocenes and post-metallocenes have been shown to give products having better comonomer incorporation and narrow molecular weight distribution. However, these catalysts often have poor high temperature stability and suffer from poor efficiencies at elevated polymerization temperatures. Additionally, the molecular weight of the polymers formed from these catalysts often decreases dramatically with increasing temperature, especially for polymers containing significant amounts of comonomer (lower density). That is, the ability of most olefin polymerization catalysts to incorporate higher α-olefins in an ethylene/α-olefin copolymer decreases with increasing polymerization temperature, due to the fact that the reactivity ratio, r1, is directly related to polymerization temperature.
Reactivity ratios of catalysts may be obtained by known methods, for example, the technique described in “Linear Method for Determining Monomer Reactivity Ratios in Copolymerization”, M. Fineman and S. D. Ross, J. Polymer Science, 5, 259 (1950) or “Copolymerization”, F. R. Mayo and C. Walling, Chem. Rev., 46, 191 (1950). One widely used copolymerization model is based on the following equations:M1*+M1M1*  (1)M1*+M2M2*  (2)M2*+M1M1*  (3)M2*+M2M2*  (4)
where Mi refers to a monomer molecule which is arbitrarily designated as “i” where i=1, 2; and M2* refers to a growing polymer chain to which monomer i has most recently attached.
The kij values are the rate constants for the indicated reactions. For example, in ethylene/propylene copolymerization, k11 represents the rate at which an ethylene unit inserts into a growing polymer chain in which the previously inserted monomer unit was also ethylene. The reactivity ratios follow as: r1=k11/k12 and r2=k22/k21 wherein k11, k12, k22 and k21 are the rate constants for ethylene (1) or propylene (2) addition to a catalyst site where the last polymerized monomer is an ethylene (k1X) or propylene (k2X).
In addition, known post metallocene catalyst compositions generally employ alumoxane cocatalysts in an amount to provide molar ratios based on metal complex from 500 to 1000 or higher or, alternatively, use cationic activating cocatalysts, such as ammonium salts of noncoordinating anions, principally, tetraalkylammonium salts of tetrakis(pentafluorophenyl)borate. These activators are expensive, especially when large amounts are necessary for catalyst activation. In addition, such catalyst compositions can result in higher catalyst residues or metal values in the polymer, which detracts from polymer properties, especially electrical properties such as dielectric properties, clarity or color (yellowness).
Thus, an olefin polymerization process is sought in which polymers containing various amounts of comonomer content can be produced with high catalyst efficiency and high monomer conversions and good reactor efficiencies without suffering from poor overall molecular weight. In addition, polymers having low molecular weight distribution or polydispersity (MW/MN<3.0) but relatively high I10/I2, are desired in such a process. Ideally, such a process could be carried out at elevated temperatures and still produce polymers having high molecular weight and relatively high comonomer incorporation as indicated by reduced density. It is known in the art that polymer molecular weight is readily controlled by use of chain transfer agents such as hydrogen or organometal compounds, such as trialkylaluminum or dialkylzinc compounds. Thus, a high temperature polymerization process that is capable of high levels of comonomer incorporation and which produces high molecular weight polymers having low molecular weight distributions and high I10/I2 values is desired in the art. Such a process additionally including a chain transfer agent to produce lower molecular weight polymers and/or incorporation of long chain branching is further desired.
In US 2005/0215737 A1, a continuous, solution, olefin polymerization process is disclosed for preparing ethylene-butene and ethylene-propylene interpolymers at high ethylene conversions. Disadvantageously, the resulting polymers were primarily plastomers having relatively low molecular weights. No chain transfer agent was employed, indicating that the inherent potential of the catalyst to produce a high molecular weight polymer was relatively low and catalyst efficiencies were also low, especially at higher reaction temperatures. It is well known that one function of a chain transfer agent is to lower the resulting molecular weight of the product at a given set of reaction conditions. Therefore, the molecular weight which a catalyst produces at a given set of experimental reaction conditions in the absence of a chain transfer agent is generally the highest molecular weight that the catalyst is capable of producing, all other conditions being equal.
For the industrial production of high molecular weight polyolefins, especially in a continuous solution process, it is especially desirable to conduct the polymerization reaction under conditions of relatively high reactor temperature, with a high conversion of the olefin monomers to polymer and having a high solids content, all with high catalyst efficiency and high comonomer incorporation (if comonomer is utilized) in the presence of a chain transfer agent. This combination of process requirements severely restricts the choice of metal complex that can suitably be employed. Metal complexes that are suited for use under less demanding conditions may, in fact, be unacceptable for use under commercial processing conditions.
In WO 99/45041, another continuous, solution olefin polymerization process is disclosed using bridged hafnocene complexes with noncoordinating anionic cocatalysts. Although the resulting polymers contained significant amounts of comonomer, catalyst efficiencies were relatively low and polymer molecular weights, even in the absence of chain transfer agent, were less than desirable.
In WO 03/102042, a high temperature, solution olefin polymerization process is disclosed using indenoindolyl transition metal complexes to prepare polyolefins at temperatures greater than about 130° C. In one example, the copolymerization of ethylene and 1-hexene was carried out at 180° C. resulting in formation of a polymer having poor comonomer incorporation (density=0.937 g/cm3) at relatively low catalyst efficiencies.
In U.S. Pat. No. 6,827,976, there are disclosed certain highly active polymerization catalysts comprising Group 3-6 or Lanthanide metal complexes, preferably Group 4 metal complexes, of bridged divalent aromatic ligands containing a divalent Lewis base chelating group. The metal complexes were employed in combination with activating cocatalysts in the polymerization of olefins including mixtures of ethylene and α-olefins, including 1-octene, to obtain polymers containing high comonomer incorporation rates at elevated temperatures.
We have now discovered that certain metal complexes may be employed in a solution polymerization process to prepare relatively high molecular weight ethylene containing interpolymers containing relatively large quantities of comonomer incorporated therein and high olefin conversions if certain process conditions are observed. Accordingly, there is now provided a process for the preparation of olefin polymer products, especially high molecular weight polyolefins, at very high catalyst efficiency. In addition, we have discovered that these catalyst compositions retain their high catalyst activity using relatively low molar ratios of conventional alumoxane cocatalysts. The use of reduced quantities of alumoxane cocatalysts (reduced by up to 90 percent or more, compared to the quantities employed in conventional processes) allows for the preparation of polymer products having reduced metal content and consequently increased clarity, improved dielectric properties and other enhanced physical properties. In addition, the use of reduced quantities of alumoxane cocatalysts results in reduction in polymer production costs. Especially desirably results are achieved by the use of certain trialkyl aluminum modified alumoxane cocatalysts. Moreover, by using alumoxane cocatalysts rather than salts of non-coordinating anions, such as tetraalkylammonium salts of tetrakis(pentafluorophenyl)borate, polymers having enhanced electrical properties, namely a decrease in 130° C. dissipation factor, can be attained.